1. Field of the Invention
The present invention relates to a vibrator that may be incorporated in a small-sized device such as a mobile cellular phone, a watch and other portable devices. More particularly, the present invention relates to a vibrator for use in a cellular phone to indicate an incoming call to a user.
2. Description of the Related Arts
Conventionally, a portable terminal device such as a cellular phone has a vibrator incorporated therein as a device that indicate an incoming call to the user by vibrations of the cellular phone body, and there has been a demand for miniaturization of the vibrator. Under these circumstances, the present applicant proposed a compact vibrator as shown in FIG. 10 (see Japanese Patent Application No. 2006-026447).
The proposed vibrator includes a voice coil 10 and a vibrating member 20 that vibrates in the axial direction of the voice coil 10 in response to the application of an alternating current to the voice coil 10. First and second suspensions 3 and 5 are disposed facing each other across the vibrating member 20 to support it vibratably in the axial direction. The vibrator further has a casing that houses the voice coil 10, the vibrating member 20, and the suspensions 3 and 5. When the voice coil 10 is supplied with an alternating current, the vibrating member 20 is reciprocated in the vicinity of a resonance frequency determined by the weight of the vibrating member 20 and the spring constant of the pair of suspensions 3 and 5, thereby generating vibrations.
FIG. 11 is a graph showing the relationship between the frequency and acceleration of the above-described vibrator. In the conventional vibrator, an input signal of a predetermined frequency in a narrow resonance frequency region t is applied to the voice coil 10, thereby obtaining vibrations with a desired amplitude. The resonance frequency region is, however, determined by the weight of the vibrating member 20 and the spring constant of the suspensions 3 and 5, as stated above. Therefore, if there are variations in the weight of the vibrator constituent elements and the spring constant of the suspensions for each vibrator, the resonance frequency region shifts in accordance with the variations. Consequently, even if an input signal of a predetermined frequency is supplied to the voice coil, the frequency of the input signal may fail to fall within the resonance frequency region t of the vibrator. In such a case, the amplitude of vibrations reduces to a considerable extent. If such a vibrator is installed in a cellular phone, it may be impossible to obtain a sufficient vibration amplitude to surely indicate an incoming call to the user.